OPEN LOOP OPTIMIZATION OF LARGE SCALE CRYOGENIC PLANTS

M. RODRIGUEZ, A. BANDONI, S. DIAZ

Rio de Janeiro, Brasil

Congreso; ENPROMER 2005; 2005

In this work, we have performed the open loop dynamic optimization of a large-scale cryogenic plant, with rigorous process models. This model includes cryogenic heat exchangers, high-pressure separation tank and distillation columns. They comprise differential energy and mass balances; hydraulic correlations and rigorous thermodynamic predictions for equilibrium calculations in main units. Special attention has been devoted to countercurrent heat exchangers, which are represented as a multicell model to transform the partial differential equations system into an ordinary differential equations system. This model also takes into account partial condensation of natural gas on shell side. The differential-algebraic equation (DAE) optimization problem is then transformed into a large-scale nonlinear programming (NLP) problem by collocation on finite elements. The resulting problem is solved using a reduced space interior point algorithm. Model resolution provides optimal temporal and spatial profiles for pressure and temperature in heat exchangers, together with partial condensation description. In the distillation column, optimal profiles have been determined for control variables (top pressure and reboiler heat duty), taking into account path constraints on product purity. The objective is to minimize the transient time to achieve an improved set point on product recovery.